I finally tried the following:
if (rvScores.FindViewHolderForAdapterPosition(prevplayer) == null)
{ rvScores.GetAdapter().NotifyItemChanged(prevplayer); }
else
{ (rvScores.FindViewHolderForAdapterPosition(prevplayer) as ScoresViewHolder).rvPoints.GetAdapter().NotifyItemChanged(prevround); }
This may be somwhat inefficient since it recreates (or recycles) the entire ViewHolder containing the inner RecyclerView, but since it is offscreen where the user can't see it I guess it isn't a big deal.